Cell types in the fetal pars tuberalis of the human adenohypophysis at mid-gestation

Melatonin as a Hormone: New Physiological and Clinical Insights

Melatonin is a ubiquitous molecule present in almost every live being from bacteria to humans. In vertebrates, besides being produced in peripheral tissues and acting as an autocrine and paracrine signal, melatonin is centrally synthetized by a neuroendocrine organ, the pineal gland. Independently of the considered species, pineal hormone melatonin is always produced during the night and its production and secretory episode duration are directly dependent on the length of the night. As its production is tightly linked to the light/dark cycle, melatonin main hormonal systemic integrative action is to coordinate behavioral and physiological adaptations to the environmental geophysical day and season. The circadian signal is dependent on its daily production regularity, on the contrast between day and night concentrations, and on specially developed ways of action. During its daily secretory episode, melatonin coordinates the night adaptive physiology through immediate effects and primes the day adaptive responses through prospective effects that will only appear at daytime, when melatonin is absent. Similarly, the annual history of the daily melatonin secretory episode duration primes the central nervous/endocrine system to the seasons to come. Remarkably, maternal melatonin programs the fetuses' behavior and physiology to cope with the environmental light/dark cycle and season after birth. These unique ways of action turn melatonin into a biological time-domain-acting molecule. The present review focuses on the above considerations, proposes a putative classification of clinical melatonin dysfunctions, and discusses general guidelines to the therapeutic use of melatonin.

Cell and molecular biology of the pars tuberalis of the pituitary

The pars tuberalis of the adenohypophysis is mainly composed of a special type of endocrine cells, pars tuberalis-specific cells, lining the primary capillary plexus of the hypophysial portal system. Dense expression of melatonin receptors and marked changes in morphological appearance, production pattern, and secretory activity during annual cycle show that these cells are highly sensitive to changes in photoperiod. This leads to the hypothesis that the pars tuberalis is involved in the transmission of photoperiodic stimuli to endocrine targets. Several investigations support the theory that pars tuberalis-specific cells are multipotential cells exerting a modulatory influence on the secretory activity of the pars distalis. Specifically, there is accumulating evidence that seasonal modulation of prolactin secretion, independent of hypothalamic input, is due to melatonin-regulated activity of pars tuberalis-specific cells. The exact nature of secretory products and their effects within neuroendocrine regulation, however, remain rather enigmatic. Accordingly, molecular mechanisms regulating gene expression under the influence of photoperiod, respectively, circulating melatonin levels are still incomplete. Recent cloning of melatonin receptor genes and new data on intracellular signal transduction will probably lead to new insights on melatonin action and pars tuberalis-specific cell physiology.

Immunocytochemistry of the pars tuberalis of the pituitary gland in some Indian wild birds: a comparative study

Immunohistochemistry has been applied to the cells of the pars tuberalis (PT) of the pituitary gland of three species of Indian wild birds (Halcyon smyrnensis perpulchra, Lonchura striata striata, Dicrurus adsimilis macrocercus). As in the pars distalis (PD), five types of immunoreactive cells (gonadotropic, GTH; thyrotropic, TSH; lactotropic/prolactin, PRL; growth hormone/somatotropic, STH; and cortico-melanotropic, ACTH/MSH cells) are present in the PT of these birds. In addition to the GTH cells, immunoreactive TSH, PRL, STH, and ACTH/MSH cells are present in the avian PT. The GTH cells are the predominant cell population of the PT. Other immunoreactive cells, though less numerous, are also present, unlike their inconsistent occurrence in several mammalian PTs. Though immunologically related, the PT cells distinctly differ from their counterparts in the PD with regard to their morphology, intensity of immunoreaction, and cellular arrangement.